Published on April 7, 2008
Oceanography Week 10Tides: Oceanography Week 10 Tides Garrison Chapter 11 Tides and forces that generate them: Tides and forces that generate them Tides are caused by the gravitational force of the moon and sun and the motion of the earth. The wavelength of tides is potentially half the circumference of the Earth. They are the longest waves. Tides are always forced waves because they are never free of the forces that generate them. Types of Tides: Types of Tides Semi-diurnal Mixed & semi-diurnal Diurnal & mixed Diurnal Tides exert uniform forces everywhere: Tides exert uniform forces everywhere The Equilibrium Theory of Tides: The Equilibrium Theory of Tides This model assumes that the ocean conforms instantly and is in equilibrium with the forces acting on it. Forces are centrifugal force and the gravity of the moon. Wrench model of two bodies orbiting their center of mass: Wrench model of two bodies orbiting their center of mass The trick is that the planet is rotating beneath the tidal bulge!: The trick is that the planet is rotating beneath the tidal bulge! Slide8: The orbital plane of the moon is tilted with respect to Earth’s axis of rotation! Tide Charts: Tide Charts Cycle times: Cycle times The lunar day is 24 h 50 m long because the moon rises 50 min later each day. The lunar cycle is 29.51 days because this completes an orbit around the Earth (period between new moons). Since the solar year is 365 d, 5 h, 48 minutes and the lunar cycle doesn’t divide evenly, each year brings new variations… Sun and moon together: Sun and moon together Variables: Variables declination of moon tilt of earth elliptical orbit of earth, moon Semi-diurnal, Diurnal, Mixed Tides: Semi-diurnal, Diurnal, Mixed Tides Dynamic theory of Tides: Dynamic theory of Tides Explains ocean tides based on sun and moon acting together (celestial mechanics) and the characteristics of fluid motion (continents). Semidiurnal tides occur twice in a lunar day. Diurnal tides occur once each lunar day. Mixed tides describe a pattern of different tidal heights throughout a tidal cycle Global Amphidromic points: Global Amphidromic points Rotation around center of mass creates orbital path for all points on Earth.: Rotation around center of mass creates orbital path for all points on Earth. Balance of Moon’s gravity and Earth’s centrifugal force creates water bulges on both sides of the planet. Tides in a Broad Basin: Tides in a Broad Basin Slide21: See also: http://www.es.flinders.edu.au/~mattom/IntroOc/notes/figures/fig11a5.html Slide22: 3. The effects of Coriolis and friction. / 4 No rotation Rotation u Add rotation fu Co-tidal lines (lines of constant phase) Co-range lines (lines of constant range) Slide23: Numerical model predictions of frictionless, rotating amphidromic system. After: Taylor, 1920. Proc. London Math. Soc., 20, 144-181. Tidal wave input Amplitude Currents Slide24: ….and friction Rotation leads to: Amphidrome, with tidal Kelvin wave entering the gulf on the right (N. hemisphere) and rotating around the amphidromic point anti-clockwise. Thus, HW propagates round the gulf. A large gulf/semi-enclosed sea will have a series of amphidromic systems, at / 4, 3 / 4, 5 / 4…etc. from the head of the gulf. Friction leads to: The outgoing tidal wave has less energy than the incoming wave (dissipated by friction), and so a smaller amplitude. The amphidromic point is shifted to the left (looking into the gulf, N. hemisphere). Tidal range on the right of the gulf is greater than on the left. In strongly dissipative systems, the amphidromic point can be shifted onto the land, becoming a degenerate amphidromic point. Tides in a narrow basin: Tides in a narrow basin Slide26: Frictionless Low friction High friction Numerical model predictions of, rotating amphidromic system with friction. After: Hendershott & Speranza, 1971. Deep-Sea Res., 18, 959-980. Low tide on a Pacific shore: Low tide on a Pacific shore Tides: change in H2O, depth, salinity, near-shore temperature Daily effects: feeding, activity Annual effects: timing of breeding, recruitment Tide pools: Tide pools Waves: high energy, turbidity in surf zone Stabilization, damage resistance, feeding strategies. Power Generation from Tides: Power Generation from Tides Tidal Friction: Tidal Friction The rise and fall of the tides and the oscillation of the Earth-Moon system consumes very large amounts of energy! Over geologic time, the length of the day and the length of the year are both being slowed by this friction. Growth rings in corals suggest that 350MYBP, a day was ~22 h and a year was ~400 d. Tidal currents: Tidal currents Flood current Ebb current Slack water Maelstrom Tidal vocabulary: Tidal vocabulary Tide gauges & databases: Tide gauges & databases Texas Coastal Ocean Observation Network NOAA Center for Operational Oceanographic Products and Services Water Level Changes Forecasts during Large Storm Events: Uses of streaming data from TCOON stations to automatically forecast future water levels The model learns changing water level patterns as the storm approaches and makes short to medium term forecasts Initially focused on Texas coastal bays and estuaries Water Level Changes Forecasts during Large Storm Events Water Levels (Flooding?) at the JFK Causeway during Storms: Water Levels (Flooding?) at the JFK Causeway during Storms Wind Approaching storms yield progressively rising water levels in the gulf Wind, precipitations and possibly other factors combine with the storm effects to possibly induce flooding of the causeway JFK Causeway: JFK Causeway JFK Causeway: only access to North Padre Island 11,000 people depend on the JFK causeway to evacuate the island Approximately 4 hours is required to evacuation Working on proposals and prototypes to forecast water levels with at least 4 hours Neural Network Forecasting of Water Levels at the JFK Causeway during 98’ Tropical Storm Frances: Neural Network Forecasting of Water Levels at the JFK Causeway during 98’ Tropical Storm Frances 4 hour DNN Forecasts Sea-level and Coasts: Sea-level and Coasts CLIMATE CHANGE DEBATE: CLIMATE CHANGE DEBATE Warmer air temperatures Warmer surface water temperatures Altered precipitation patterns/hydrological patterns Altered weather and climate (hurricanes, storms, etc.) CLIMATE CHANGE IMPACTS ON OCEAN: CLIMATE CHANGE IMPACTS ON OCEAN INCREASES IN SEA LEVEL AND SEA-SURFACE TEMPERATURE (AFFECTING HURRICANES, NORTHEASTERS) DECREASES IN SEA-ICE COVER CHANGES IN SALINITY, ALKALINITY, WAVE CLIMATE, AND OCEAN CIRCULATION HURRICANES, STORMS, AND WAVES: POSSIBLE CHANGES TO FREQUENCY, INTENSITY, AND PATHS OF HURRICANES MODELING DATA DO NOT SUPPORT INCREASES IN HURRICANES AND WAVES LITTLE INFORMATION ON NORTHEASTERS HURRICANES, STORMS, AND WAVES RELATIVE SEA-LEVEL CHANGES: RELATIVE SEA-LEVEL CHANGES RELATIVE SEA-LEVEL CHANGES DUE TO LAND-LEVEL CHANGES AND WATER-LEVEL CHANGES LAND-LEVEL CHANGES RESULT FROM TECTONICS POST-GLACIAL REBOUND RSL (CON’T): RSL (CON’T) WATER LEVEL CHANGES RESULT FROM: STERIC EFFECT (INCREASED WATER TEMPERATURE AND LOWER SALINITY) EXCHANGE OF WATER WITH GLACIERS, ICE-CAPS (EARTH RHEOLOGY, ROTATION) HUMAN ACTIVITIES (WATER STORAGE SUCH AS GROUNDWATER, LAKES) RSL: MEASUREMENTS: RSL: MEASUREMENTS HOW DO WE MEASURE RELATIVE SEA LEVELS? DATING BURIED COASTAL VEGETATION (SALT MARSHES, MANGROVES, ETC.): SPARSELY DISTRIBUTED TIDE GAUGES: MOST IN MID-LATITUDE NORTHERN HEMISPHERE, FEW IN MIDDLE OF OCEANS, CONTAMINATED BY EARTH MOVEMENTS RSL HISTORY: RSL HISTORY RELATIVE SEA-LEVEL RISE IMPACTS: RELATIVE SEA-LEVEL RISE IMPACTS Lowland inundation and wetland displacement Shoreline erosion More severe storm-surge flooding Saltwater intrusion into estuaries and freshwater lagoons Altered tidal range in rivers and bays Changes in sedimentation patterns Elevated sea-surface and ground temperatures FUTURE RSL: FUTURE RSL RSL UNCERTAINTIES: RSL UNCERTAINTIES MODELS SHOW ACCELERATION IN RSL DURING 20TH CENTURY; DATA DON’T MODELS UNDERPREDICT RSL IN THE 20TH CENTURY, COMPARED TO OBSERVATIONS Delta morphology : Delta morphology Deltas form as a result of sediment deposition from river inflow and dispersion by ocean processes. Deltas may be River, Wave, or Tide Dominated. Primary Coasts: © 2002 Brooks/Cole, a division of Thomson Learning, Inc. Primary Coasts What are some types of primary coasts? Land erosion coasts Coasts built out by land processes Volcanic coasts Coasts shaped by earth movements Primary coasts: Primary coasts Glacial debris formed most of the islands in Cape Cod. Rocks and loose soil (till) was deposited when the glaciers advanced, then retreated. What does this indicate about former sea levels. Secondary Coasts: © 2002 Brooks/Cole, a division of Thomson Learning, Inc. What are some processes that shape secondary coasts? Waves and currents Stream erosion Abrasion of wind-driven particles Freeze/thaw cycles Slumping Secondary Coasts Secondary coasts: Secondary coasts Elephant Rock: Elephant Rock Beach material: Beach material A beach is a zone of loose particles that covers a shore. Dunes, berms, & bars...: Dunes, berms, & bars... What are the features of a beach? berm berm crest backshore foreshore beach scarp longshore trough longshore bars Coastal cells: Coastal cells Sections of coast in which sand input and sand output are balanced are referred to as coastal cells. Bays & Barrier Islands are large-scale features of secondary coasts: Bays & Barrier Islands are large-scale features of secondary coasts Coasts Formed by Biological Activity: © 2002 Brooks/Cole, a division of Thomson Learning, Inc. Coral Reefs are the most dramatic of the coasts formed by biological activity. Coasts Formed by Biological Activity Estuary: Estuary Slide64: Figure 1 Regional comparison of hydrographic and tide-gauge-measured sea level change in the Eastern Pacific. Hydrographic profile observations of temperature and salinity converted into 1,000-m dynamic height anomalies (green), their 5-yr running means (red data points) and linear regression (black) compared with 5-yr running mean relative sea levels from tide gauge observations at San Francisco, San Diego, Honolulu and Balboa (purple lines). Tide gauge series have been vertically offset to coincide with the earliest dynamic heights. Map inset shows tide gauge locations in purple and observed dynamic height locations in green. Slide65: Figure 2 Local comparison of hydrographic and tide-gauge-measured sea level change. a, As in Fig. 1, but hydrographic observations limited to 4,400 km 1,100 km area adjacent to gauge sites at San Francisco and San Diego; b, to 1,100 km 1,100 km area centred on gauge site at Honolulu; c, to 800 km 1,400 km area adjacent to gauge site at Balboa, Panama. Slide66: Figure 3 Regional comparison of hydrographic and tide-gauge-measured sea level change in the Eastern Atlantic. As in Fig. 1, but hydrographic observations limited to 1,400 km 2,200 km area adjacent to tide gauge sites at Cascais (Portugal) and Tenerife (Canary Islands). Slide67: Figure 4 Regional comparison of hydrographic and tide-gauge-measured sea level change in the Western Atlantic. As in Fig. 1, but hydrographic observations limited to 1.300 km 500 km triangular region adjacent to tide gauge sites at Boston, Portland and Halifax. Time series of dynamic height anomalies computed from the WOA98v2 interpolated data set are shown in blue, offset by 0.4 m. Map inset shows tide gauge locations in purple, observed dynamic height locations in green and interpolated dynamic height locations in blue.
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